1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of geographic areas that are often referred to as cells. Mobile units located in each cell may access the wireless communications system by establishing a wireless communication link, often referred to as an air interface, with a base station or access network associated with the cell. The mobile units may include devices such as mobile telephones, personal data assistants, smart phones, Global Positioning System devices, wireless network interface cards, desktop or laptop computers, and the like. Mobile units may also be referred to as mobile nodes. Communication between the mobile unit and the access network may be authenticated and/or secured using one or more authentication protocols.
A mobile node that has an established security association with a first access network may roam to another cell served by a second access network. In mobile networks such as WiMAX networks or CDMA2000® High Rate Packet Data (HRPD) networks defined by the Third Generation Partnership Project (3GPP, 3GPP2) active sessions are transferred from one access network to another by conducting Mobile IP re-registrations, which are defined in the IETF Standards RFC 3012 and RFC 3344. In a typical mobile IP re-registration, a mobile node (MN) generates a Registration Request (RRQ) when the mobile node moves from one access network to another. The originating access network is represented by a Foreign Agent (FA), which forwards the Registration Request to the mobile node's Home Agent (HA) located in the Core Serving Network (CSN). The Registration Request message therefore provides a binding between the mobile node, the foreign agent, and the home agent that can be used to deliver user traffic to the mobile node while the mobile node is in motion.
Unauthorized or fraudulent mobile nodes may attempt to gain access to the wireless communication system by establishing a false binding between the unauthorized mobile node, a foreign agent, and a home agent associated with an authorized mobile node. In order to avoid false bindings requested by unauthorized mobile nodes, each Resource Request contains an authentication signature that can be used to authenticate an existing security association between the authorized mobile node and the home agent. This signature is typically included in a portion of the Resource Request message that may be referred to as the MN-HA Authentication Extension (MN-HA-AE) of the RRQ message. The MN-HA-AE is computed according to the RFC 3344 using a secret key established between the authorized mobile node and the home agent. This secret key is typically referred to as the MN-HA key.
Each home agent may store numerous MN-HA keys associated with different mobile nodes. Accordingly, the authentication extension MN-HA-AE can include an index, known as the Security Parameter Index (SPI), which indicates the particular instance of the MN-HA key that should be used to authenticate the security association between the mobile node and the home agent. When the home agent receives a resource request message, the home agent extracts the SPI from the authentication extension MN-HA-AE. The home agent then checks the SPI value against its database of active security associations for the mobile node. If the home agent finds the extracted SPI value in the database of active security associations, the home agent uses the associated MN-HA key to validate the authentication extension MN-HA-AE. However, the database may not include the SPI that was transmitted in the authentication extension. The home agent may therefore request the MN-HA key from an Authentication, Authorization, and Accounting (AAA) server associated with the mobile node if the home agent is unable to locate the SPI in the database of active security associations. The home agent may record the key received from the AAA server alongside the SPI in the database of security associations and use the key to validate the authentication extension MN-HA-AE.
Existing standards do not define consistent procedures for provisioning the keys that are used in the authentication extension MN-HA-AE. In CDMA2000® High Rate Packet Data (HRPD) networks, the MN-HA keys are pre-provisioned and do not change for the lifetime of the mobile node. In other networks, the MN-HA keys can be determined using other pre-provisioned keys and pre-defined secure algorithms. In WiMAX systems, the MN-HA keys are determined by bootstrapping from an active access security association, which is established using Extensible Authentication Protocol (EAP) transactions. For example, the Enhanced Master Session Key (EMSK) can be generated as the result of a successful EAP authentication. The EMSK is used as the secret seed to create a Mobile IP Root Key (MIP-RK) and the MIP-RK is used to create the hierarchy of necessary Mobile IP security associations for Mobile IP, including MN-HA key. This key hierarchy is shown in FIG. 5.2 of the WiMAX Stage 3 Description (this figure is reproduced herein as FIG. 1). If the mobile node does not support the Mobile IP protocol, and the access network may act as a proxy Mobile IP client or proxy Mobile IP node. In this case, a PMN-HA key may be generated for the proxy client or node using the EMSK. A new EMSK is generated when a mobile node traverses a boundary between two un-trusted access networks and so a new set of Mobile IP keys, including the MN-HA and PMN-HA keys, is created.
Since the MN-HA and PMN-HA keys are not provisioned in a consistent manner, each home agent may have a more than one MN-HA and/or PMN-HA key for each of the mobile nodes. Consequently, it may be difficult for the home agent to determine whether it has the correct MN-HA or PMN-HA key for validating the authentication extension transmitted by a mobile unit or whether it should request a new MN-HA or PMN-HA key from the AAA server. The home agent may also have difficulty determining which key is being requested by the authentication extension, particularly if the database of active security associations for the mobile node indicates that multiple MN-HA or PMN-HA keys have been established for different security associations in different types of network. These problems are exacerbated by the fact that existing standards do not define procedures for selecting SPI values for the MN-HA or PMN-HA keys. For example, the same SPI values may be generated for different MN-HA or PMN-HA keys, leading to collisions when the corresponding mobile nodes transmit authentication extensions.